This invention relates to a method of nondestructive testing. More specifically, this invention relates to a method for the non-destructive detecting of defects such as micro-cracks in the surface of objects made of ceramic materials
There is a need to develop materials like ceramics for high-temperature applications, such as for turbine engine components. Ceramic materials that do not necessarily require cooling can be used to decrease fuel consumption and improve engine operating efficiencies. Furthermore, ceramic materials are often less costly then metals, more corrosion resistant, and are normally fabricated from abundant elements.
Nondestructive evaluation techniques such as ultrasonic, penetrating radiation or optical techniques are capable of detecting flaws such as cracks, porosity and nonbonding in ceramic components as well as variations in material properties such as density and elastic moduli. The application of these techniques can help ensure reproducible mechanical and physical properties, thus improving ceramic processing techniques and operational reliability.
Flaws approaching critical sizes in ceramics can be detected in geometrically simple objects under ideal conditions. Critical flaw sizes may vary with the particular material but have been estimated to be about 10.mu. in hotpressed silicon nitride and about 100.mu. in reaction bonded components. While flaws in geometrically simple objects can be detected under ideal conditions, components with complex geometries still cannot be inspected adequately.
Ceramic components may fail by the propagation of existing flaws to critical size. Thus determination of initial defect distribution and flaw characteristics in a component is important. The defects that are necessary to detect are pores, inclusions, cracks and large grains. Flaws with dimensions of 10 to 100.mu. must be detected in ceramic components such as silicon nitride if they are to operate as structural materials in high-temperature environments, i.e. up to 1400.degree. C., and at stresses up to 300 MPa. This flaw size is a few orders of magnitude smaller than critical flaw sizes in metal.
Methods for detecting micro-cracks which were investigated included the use of ultrasonics, acoustic emission, holographic interferometry and acoustic microscopy. None of these methods were found to be completely satisfactory. For example, it is not possible to consistently detect the presence of micro-cracks in the components. Furthermore, the cost of handling and applying the detection method, when thousands of individual pieces must be inspected, is prohibitive.
Other methods for detecting surface defects include the use of surface penetrants containing dyes and the use of radiation. Penetrants, however, are generally not capable of resolving flaws with critical dimensions less than about 300.mu. and provide no information at all concerning flaw depth. The resolution of conventional X-ray systems is about 100.mu., which can, however, be improved with more sophisticated electron focusing techniques. The biggest problem with X-rays is image contrast which is determined by the relative values of the absorption coefficients of the flaw and the material. In general, microcracks, i.e. cracks down to about 10.mu. wide in ceramic materials, are difficult if not impossible to detect, since the X-ray beam must be exactly parallel to the crack plane in order to see the flaw.